In our proposed research project, there are two hypotheses that we will test: (a) the intestines of the fetus and neonate are able to adequately provide for their oxidative requirements when oxygen availability and oxygen demand change over a wide range, and (b) adjustments in oxygen extraction, rather than blood flow, represent the primary means by which the intestines maintain aerobic metabolism as O2 availability and O2 demand fluctuate. We propose to assess the adequacy of intestinal oxygenation during decreases in O2 availability (due to hypoxemia, isovolemic anemia, and decreasing perfusion pressure), increases in O2 availability (due to icovolemic polycythemia), increases in O2 demand (induced by digestion), and decreases in O2 demand (resulting from the use of total parenteral nutrition. All studies will be performed on chronically-catheterized fetal and neonatal lambs. Intestinal blood flow will be measured with the radionuclide-labelled microsphere technique, while intestinal O2 extraction and O2 comsumption will be calculated with modifications of the Fick equation. The adequacy of intestinal oxygenation will be established by determining if the intestines produce anaerobic metabolites when O2 availability or O2 demand change. The results of our studies will identify the limits whithin which the fetal and neonatal intestinal tracts maintain aerobic metabolism when subjected to various stresses that can commonly occur during the perinatal period. The stressful transition that takes place naturally at birth can have important effects on a gastrointestinal system that must provide for the entire nutritional needs of the rapidly growing newborn - a task not required of the fetus in utero. From our observations on the control of intestinal oxygenation in the fetus and neonate, we hope to establish a unifying concept for the regulation of oxygenation that can be applied to several other organs whose functions change at birth.